1. Field of the Invention
The present invention is directed to laser recorders for producing images on a substrate, and in particular, to laser recorders which are capable of printing with grey scales.
2. Description of Related Art
It is widely recognized that printing with grey scale techniques can offer substantial advantages in terms of image storage and process speed over high resolution bitmap techniques. The ability to create precisely intensity controlled levels in a laser is also important for the realization of novel schemes for high addressability in laser printing. This is particularly the case in the printing of half tone images where the required resolution necessitates extremely high resolution bitmap images, or the application of enhanced addressability techniques.
One issue of considerable concern in the realization of light sources for grey scale printers and high addressability printers is the precise control of the intensity of the (typically laser) light source to provide for precise levels of grey or precise edge position control. This becomes a problem of increasing severity as the number of levels desired increases.
Most techniques for varying intensity rely on precisely varying the forward bias current to the light emitting device in order to vary the light emission. In this scenario, shifts in the L-I characteristic of the device result in a need to re-calibrate the current levels needed for each of the intensity levels.
An example of a laser beam printer which provides grey scales by varying the forward bias current is provided in U.S. Pat. No. 4,727,382 to Negishi et al. The laser printer disclosed in U.S. Pat. No. 4,727,382 includes a controller which receives an N-bit word which is used to look-up a forward bias current value in a ROM. The forward bias current level found in the ROM is then supplied to the laser so that the intensity of light produced by the laser is controlled based upon the value of the N-bit input word. A feedback circuit is also provided.
U.S. Pat. No. 4,384,297 to Ohara et al discloses a laser recorder for producing a picture having half-tones of high accuracy, wherein an analog input video signal is sampled at a predetermined frequency. Each time the input video signal is sampled, a burst of high frequency pulses are output to drive a laser. Each burst of high frequency pulses is output synchronized with each pixel time so that the number of high frequency pulses output in each burst controls the intensity of light output by the laser. The high frequency pulses all have the same amplitude, and are centered about the sampling pulse (within each pixel time).
U.S. Pat. No. 4,384,297 requires each burst of high frequency pulses to be synchronized with the video signal so that each burst of high frequency pulses is centered within a pixel frame. This requires synchronization circuitry, which adds to the complexity of the control system. Additionally, because the high frequency pulses are not generated on the same semiconductive substrate (chip) as the laser, off-chip high frequency connections between the laser and the high frequency pulse generator are required. Off-chip generation of high frequency pulses creates difficulties due to lead inductance and parasitic capacitance.
Other patents of interest include U.S. Pat. No. 4,539,685 to Hart et al and U.S. Pat. No. 4,893,136 to Curry. U.S. Pat. No. 4,539,685 discloses a passively Q-switched laser having a variable pulse rate wherein the duration of the laser power pulse and the time interval between laser power pulses are variable. A detector is provided for detecting an output laser pulse and feeding it back to a controller. U.S. Pat. No. 4,893,136 discloses an arithmetically computer motor hunt compensation system wherein a variable frequency clock controls a buffer which is receiving data samples. The data samples are representative of individual dots of a halftone image.
U.S. Pat. No. 4,987,468 to Thornton and U.S. Pat. No. 5,038,185 to Thornton disclose techniques for integrating transistors, detectors, and lasers onto a common semiconductive substrate.
In particular, U.S. Pat. No. 4,987,468 discloses a lateral heterojunction bipolar transistor (LHBT) which can also be used as a hetero transverse junction (HTJ) laser. See, for example, col. 2, lines 53-63 of that patent. The LHBT includes emitter and/or collector regions forming p-n heterojunctions at the emitter/base junction and at the collector/base junction with a planar base region, wherein at least the emitter region is formed by employing impurity induced disordering (IID) to produce an emitter or collector region of wider bandgap than the base region. When the collector p-n junction is forward biased (i.e., is driven into saturation by forward biasing both the emitter/base and the collector/base p-n junctions or sufficiently forward biasing one of these junctions) the carrier density in the base region builds to sufficiently high levels to exhibit stimulated emission gain and bring about lasing action. The intensity of the light beam output by the laser can be varied by sufficiently forward biasing one of the emitter/base and collector/base junctions to bring about lasing (e.g., by supplying a current in the 6-10 mA range) and by modulating a current in the nA to mA range and modulating the voltage (e.g., 0-1.2 volt) supplied to the other one of the emitter/base and collector/base junctions to modulate the emission properties (intensity) of the transistor/laser. See col. 5, line 1-col. 6, line 23 of that patent.
U.S. Pat. No. 5,038,185 discloses transistors and lasers integrated in a common epitaxial layer of a semiconductive substrate. More specifically, compound semiconductor (such as AlGaAs) surface skimming hetero- transverse junction (HTJ) lasers and improved heterojunction bipolar transistors (L-HBTs) are disclosed. The surface skimming characteristic of these HTJ lasers enables them to be configured relatively easily, without requiring any epitaxial regrowth, to operate as distributed feedback (DFB) lasers in axial or surface emitting configurations and to function as harmonic generators. The device acts as a transistor when its collector/base junction is reverse biased and its emitter/base junction is forward biased. The device acts as a single quantum well HTJ laser when its collector/base and emitter/base junctions are both forwardly biased sufficiently to cause the carrier density in its active base region to build to a level at which stimulated emission is initiated. The possibility of optoelectronic integration with the laser is also recognized, although no circuitry is disclosed. See col. 3, lines 25-35 and col. 8, lines 35-38.
The disclosures of U.S. Pat. Nos. 4,987,468 and 5,038,185 are incorporated herein by refernce. However, neither of these references discloses the grey scale or power control circuitry provided by the present invention.
U.S. Pat. No. 5,048,040 to Paoli, the disclosure of which is incorporated herein by reference, discloses a multiple wavelength semiconductor laser. Several lasers operating at different wavelengths are integrated into a single chip. See col. 1, lines 53-63.